Hydrogen Bonding in the 2,2'-Bipyridinium Salt of 1,2,4,5-Benzenetetracarboxylic Acid (Pyromellitic Acid)

The 2,2'-bipyridinium salt of pyromellitic acid (PMA) (1,2,4,5-benzenetetracarboxylic acid), 2,2'-bipyridinium hemi[1,2,4,5-benzenetetracarboxylate(2-)] hemi(1,2,4,5-benzenetetracarboxylic acid), C₁₀H₈N₂H⁺.1/2[C₆H₂(COO)₄H₂O]^2-.1/2[C₆H₂(COO)₄H₄], has been prepared and studied by X-ray diffraction and IR spectroscopy. 2,2'-Bipyridine acts as a proton sponge by accepting a proton from pyromellitic acid. The transfer of protons results in strong asymmetric intramolecular hydrogen bonds: H(N)...N 2.603 (2) Å in the bipyridinium cation which has syn conformation and H(O)...O 2.396 (2) Å in the pyromellitate anion. The N-H...N hydrogen bond is part of a three-centred hydrogen bond with a carbonyl group of the neutral pyromellitic acid: N-H...N,O with H(N)...O 2.805 (2) Å. Each neutral PMA molecule is connected by hydrogen bonds with two cations and four anions forming a two-dimensional network with plane indices (112). The IR spectrum also suggests the presence of strong hydrogen bonds.

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